This invention relates to the offshore production of hydrocarbon fluids and more particularly but not exclusively is concerned with means for assisting the extraction of hydrocarbons by pumping a displacement fluid directly or indirectly into the submarine source.
According to one aspect of the present invention, there is provided an apparatus for offshore production of hydrocarbon fluids, comprising a base and a buoyant structure secured to said base and connected or adapted to be connected to a subsea well or wells, characterised in that: (A) said buoyant structure is tethered to said base by a plurality of flexible tethers; (B) a flexible riser is provided between said base and said buoyant structure; and (C) said buoyant structure comprises a submerged section comprising means for providing a displacement fluid under pressure to an output which communicates with said base via said flexible riser.
Advantageously, the base is adapted to function as a storage vessel. The buoyant structure can comprise an upper section which extends above the sea level.
In another aspect, the invention provides a method of extracting hydrocarbons from a submarine well, which is characterised in that a displacement fluid, e.g. water, is injected under pressure into the submarine well, or into a reservoir connected to the submarine well, from a submerged buoy or from a submerged part of a buoy.
Conveniently, the displacement fluid is rawwater (untreated or minimally treated seawater). Optionally, the rawwater can be treated prior to use; normally such treatment will involve a deoxygenation process.
According to another aspect of the present invention, there is provided a buoy which comprises an upper section arranged to extend above the sea level; and a lower, submerged section, characterised in that the lower, submerged section comprises means for providing a displacement fluid under pressure to an output which is connected or is connectable to a submarine well.
In the apparatus and buoy as defined above, the means for providing a displacement fluid is preferably a pump or a series of pumps capable of delivering fluid at an output pressure in the range of 500 to 5,000 psi. The presently preferred operating pressure is 3,000 psi.
Power for operating the pump(s) may be provided by a generating system mounted in or on the buoyant structure.
The connection to the submarine well is preferably by means of one or more flexible risers. These may be connected directly to the wellhead, e.g. through a water injection manifold or a tree flowbase, or via a storage structure located at or close to the wellhead. Preferably the displacement fluid is water, e.g. rawwater or deoxygenated rawwater.
In one embodiment, the buoy of this invention provides pressurised water for injection into two or more submarine wells, e.g. via a pair of flexible risers which diverge at or close to the seabed to reach the submarine wells.